1. Field of the Invention
The invention relates to a method and device for the projection of a color image resulting from a modulation by optical valves. It can be applied notably to the projection of an image on a wide screen or to the projection of an image for assistance in the driving or piloting of vehicles such as automobiles or aircraft.
2. Description of the Prior Art
The standard method of projecting a color image consists in using three optical valves to modulate three corresponding color beams that respectively represent the three basic components of color which, in a common example, are red, green and blue. The three color beams are obtained by the direct or successive separation or splitting of a primary beam of white light that is depolarized and collimated. According to the successive separation, the primary beam is first of all separated into two depolarized secondary beams, one representing one of the three color components and the other representing the other two components which are then separated into two beams. Thus, three depolarized color beams are obtained.
The optical valves respectively receive the three electrical signals that respectively represent the same three basic color components of an electrical color video signal. They modulate the three respective color beams by rotation of the polarization of the incident light flux. The modulation therefore necessitates the preliminary polarizing of the three color beams, then an analysis of the polarization. Each of the three valves is therefore placed between an absorption linear polarizer and a linear analyzer of polarization. In practice, it has been found that absorption polarizers cause the loss of about half of the light energy of the three depolarized color beams. Taking the other losses in the projection device to be negligible, the color image resulting from the combination of the three modulated color beams has an energy value that is twice as small as the energy of the primary beam of white light.
It follows from this that a standard projector implementing this method of projection of a color image comprises a generator of a beam of white light that is depolarized and collimated, a separator of the primary beam into secondary beams, three optical valves for the respective modulation of three color beams respectively representing three basic color components and coming from the secondary beams, and means to combine the three color beams to constitute a composite color beam representing the color image.
The generator of the primary beam is commonly formed by a single or composite source of depolarized white light and a collimation device. The valves are liquid crystal matrix plates, each gripped between a first absorption linear polarizer and a polarization analyzer, ordinarily formed by a second absorption linear polarizer.
For the rest of the description, it must be noted that there are two sorts of means of suppression, namely the means for the suppression, from an incident radiation, of a spectral band, either by absorption (filtration) or by deflection direction in a direction (multidielectrical mirror) and means for the suppression, from an incident radiation, of a direction of polarization, either by absorption (polarizer) or by deflection in a direction of propagation (multidielectrical mirror). These means of suppression are arranged so that the luminous intensity of each of the three color beams constituting the projected image comes only from one of the three corresponding optical valves.
Standard projectors currently raise two major problems. The first problem pertains to their reliability which is directly linked to the reliability of the light source. Their reliability is increased by the incorporation therein of an auxiliary or back-up lamp, activated in the event of a malfunction of the main lamp. Naturally, the generation of the back-up primary beam should meet the same constraints as those pertaining to the main primary beam. Both the primary beam generators should therefore be installed fixedly in a projector. Up till now, the installation of back-up projectors integrates poorly with the standard projection devices and gives a color image of lower quality than the one obtained from the main lamp.
For example, a known way of making a projector consists in separating the main primary beam by means of two multidielectrical mirrors. The solution found for the back-up illumination consists in placing the back-up lamp behind the second mirror. This solution has the major drawback of illuminating two valves by two color beams that are different from those coming from the main lamp and of not illuminating the other valve. It is therefore not possible to reproduce all the hues satisfactorily with the back-up installation.
The second problem relates to the adapting of the luminous intensity of the projected image to the ambient light conditions, notably to day-time and night-time conditions. To make the image highly luminous by day and to give it low luminosity by night, the luminous power of the projected image should be capable of varying greatly and, at the same time, it should be possible to keep the same geometrical and spectral quality of the image. The document EP-A-0 246 128 presents the difficulties of making substantial modifications in the luminous intensity of the source of the primary beam of white light and proposes, as a solution, a device for the modulation of the primary beam after collimation. This complex device is therefore added to the structure of the projector and adds to its cost.
Furthermore, the compatibility of the projection devices with night vision equipment imposes a particular emission spectrum on these devices for night-time use. It is indeed necessary to attenuate the red and infrared part of the spectrum emitted at night. This constraint would therefore make it necessary to further add a particular device to modify the spectrum of the primary beams emitted, this device being activated only for night-time projection.
The invention resolves these two problems raised by the standard projection method and the projection devices that result therefrom. Through the invention, the back-up primary beam can give an image having a quality equal to that obtained with the main primary beam. Furthermore, the invention can offer both types of projection, namely projection adapted to day-time conditions and projection adapted to night-time conditions. All these advantages are obtained not by a complication of the method and of the standard projection devices but through a novel method of projection that is as simple as the present method and through the projection device implementing this method.